The volume of communication information data tends to greatly increase with the advancement of the information society. Investigations have been vigorously performed to increase the capacity of data transmission. Consequently, increase in the capacity of a wavelength division multiplexing (WDM) optical transmission system (increase of the number of channels, expansion of a transmission band, etc.) is under development.
WDM transmission is a system transmitting and receiving WDM signals of a plurality of wavelengths through one optical fiber, and for increasing the capacity of the WDM transmission, it is indispensable to realize a wide band optical fiber amplifier, amplifying the signals.
The optical fiber amplifier using an Erbium (Er) doped optical fiber plays an important role as a key device in WDM system.
The conventional Er-doped optical fiber comprises of a core 1 and a cladding 5 having a refractive index smaller than that of the core, as shown in FIG. 9(A). The refractive index profile of the core 1 is a step index type profile, and it is doped with the rare earth element Er. In addition, as shown in this figure, “a” shows the diameter of the core 1 and is defined as the diameter of the core at one-tenth maximum of refractive index.
The amplification band of an Er-doped optical fiber accords with the 1550 nm wavelength band where transmission loss of the transmission fibers becomes lowest. Moreover, the Er-doped optical fiber has high amplification efficiency in the amplification band, even if the host glass silica is doped with Er, wherein the amplification band of the Er-doped optical fiber covers the wavelength band of 1530 nm˜1560 nm, called as C-band.
In addition to the above-mentioned C-band, the wavelength band of WDM transmission light has been further expanded even to the wavelength band of 1570 nm˜1600 nm, called as L-band, in recent years.
Although a conventional Er-doped optical fiber developed for C-band is applicable to L-band, its gain per unit length in L-band is smaller than that in C-band. Therefore, in order to obtain a gain in L-band equivalent to that in C-band by using the conventional Er-doped optical fiber, longer length, by several times of the Er-doped optical fiber, is necessary.
Moreover, the increase in the number of channels accompanying increase of the capacity of WDM transmission leads to the increase of the signal light intensity input into an Er-doped optical fiber. Therefore, an Er-doped optical fiber is required for a higher saturation output power.
However, the increase of the length of Er-doped optical fiber and the increase of the signal light intensity input to an Er-doped optical fiber cause adverse nonlinear effects in an Er-doped optical fiber amplifier such as four-wave mixing (FWM), the cross phase modulation (XPM), etc.
In order to suppress such nonlinear effects, it is effective to increase the gain coefficient (gain per unit length) of the Er-doped optical fiber. A gain coefficient can be expressed by the following equation (1).
 G(λ)=α(λ)·[n2·{σe(λ)/σa(λ)+1}−1]  (1)
Here, λ is the wavelength, G(λ) is a gain coefficient having the unit of dB/m and α(λ) is an absorption coefficient of the rare earth element doped optical fiber for optical amplification, and is the absorption coefficient of Er-doped optical fiber in this case. The unit of the absorption coefficient is dB/m.
σa(λ) is the absorption cross section, σe(λ) is the stimulated emission cross section and n2 is the ratio of upper laser level's Er density to total Er density. The gain coefficient, the absorption coefficient, the absorption cross-section, and the stimulated emission cross section have wavelength (λ) dependence.
The ratio of the stimulated emission cross section and the absorption cross section, in equation 1, is determined according to the host glass, and n2 is determined by the excitation conditions (population inversion). Therefore, in order to raise a gain coefficient, it is necessary to increase the absorption coefficient α(λ).
This absorption coefficient is proportional to the density of Er ions as well as the overlap integral between the Er ions distribution and the optical mode envelope. Therefore, in order to enhance the gain coefficient in an Er-doped optical fiber, increasing the Er dopant concentration and/or enlarging the overlap integral is being taken up.
Moreover, enlargement of the value of chromatic dispersion can also suppress the nonlinear effects. It is known that the generation of four-wave mixing will increase rapidly due to phase matching, if zero dispersion wavelengths exist in a transmission wavelength region. Therefore, in order to reduce the generation of four-wave mixing, generally the absolute value of the chromatic dispersion in transmission wavelength is increased greatly, and kept them away from phase matching conditions.
However, in the conventional Er-doped optical fiber, owing to the concentration quenching, a limitation to the doping density concentration of the Er ions exists. Concentration quenching is a phenomenon, which produces energy loss because, as the concentration of the dopant element, Er in the present case increased the distance between Er ions is shortened and the ion-ion interactions occur.
In the case of Al2O3—SiO2 host wherein Aluminum is co-doped to suppress the effect of concentration quenching, a reduction in the conversion efficiency occurs, when the dopant concentration of Er exceeds 1000 wtppm, due to the concentration quenching. Therefore, the Er concentration cannot be increased drastically over 1000 wtppm though it is chosen from a general balance of the decline of this conversion efficiency, increase in the absorption coefficient, etc.
On the other hand, shifting the cutoff wavelength to a long wavelength side and doping Er throughout the core part increases the overlap integral between the Er ions distribution and the optical mode envelope. In order to shift the cutoff wavelength to a long wavelength side, it is effective to enlarge the core diameter.
However, the cutoff wavelength must be shorter than the pump or signal wavelength to ensure the single mode propagation. Therefore, the above mentioned overlap integral enlargement, by lengthening the cutoff wavelength, also has limitations.
As mentioned above, in the conventional Er-doped optical fiber, because of the limitations on the overlap integral enlargement and also on the Er doping concentration, a limit existed to the enhancement of the gain coefficient by increasing the absorption coefficient.
Moreover, in an Er-doped optical fiber, the relative refractive index difference of the core to the cladding and the cutoff wavelength are determined from a viewpoint of improvement in the amplification characteristics. And since chromatic dispersion is uniquely decided by the relative refractive index difference and the cutoff wavelength, the flexibility of the adjustment is low. Therefore, in the conventional Er-doped optical fiber, a limit existed also in the enlargement of the absolute value of chromatic dispersion.